Gain and power output of a travelling-wave tube are heavily dependent on the cathode current thereof. When, as is often the case, the tube is used in a radar installation for generating high-frequency pulses with high effect by grid control with voltage pulses, it can also be that the length and power of the generated pulses will also depend on the tube cathode current. When gain is too great, high-frequency oscillations can also occur due to saturation phenomena in the emitted radar pulses.
In its turn, the cathode current is dependent on the tube temperature, or more explicitly, variations in the grid-cathode distance which occur during warming-up, this distance first decreasing and then assumes its nominal value on attaining a constant temperature. The warming-up period can have a duration of up to two hours, the variation being heaviest during the first thirty minutes.
Part of the electrons emitted by the cathode and forming the cathode current srike the structure of the tube and give rise to an electric current in the metal parts thereof, a so-called body-current. If the cathode current increases beyond a certain limit during the heating-up period, such body current gives rise to considerable heating of the metal parts, so that the current must be interrupted before the tube is damaged. This problem cannot be accepted in a radar station.